There are various methods and devices, known in the prior art, for monitoring and analyzing a signal transmitted via a communication link, in order to measure or estimate various parameters of transmitted signals which could be of interest to those performing diagnostics and maintenance of communication networks.
Some of such methods allow determining presence and/or quality (often indicated as Q) of a digital data signal transmitted via a communication line. Needless to say, that knowledge about the presence and quality of data signals is highly important for operation of communication systems, especially of multi-channel ones. Modem multi-channel communication systems include optical, wired and also wireless systems.
Conventional methods monitor quality of an optical/non-optical signal solely by the intensity of the signal.
Another widely known method for determining quality of a digital data signal is by measuring its so-called bit error rate (BER).
U.S. Pat. No. 6,570,682 describes a method performing fault management in an optical communications system including a polarization mode dispersion (PMD) compensator. The method and apparatus utilizes a controller receiving notifications indicative of PMD in the system, as well as at least one indicator from a system device providing an indication of an attribute of the communication system. Such system devices include a receiver providing a Q (quality) factor and a bit error rate (BER) of an optical signal, and a signal-to-noise ratio meter providing an SNR of the optical signal. The method intelligently provides fault management by correlating the PMD notifications and system indicators to distinguish between fiber failure, PMD-related degradations, and failure of monitoring equipment itself.
U.S. Pat. No. 6,583,903 describes an optical data communications link where a modulated optical signal is emitted from an optical transmitter, coupled into one end of an optical fiber, and received by an optical receiver at the opposite end of the fiber. The U.S. Pat. No. 6,583,903 relates to reducing the polarization-mode dispersion (PMD) of the modulated optical signal, which tends to limit the usable bandwidth of the fiber. A polarization controller is applied to the modulated optical signal substantially near the transmitter. The receiver measures the quality of the received signal and issues commands to the polarization controller to improve the received signal quality. In various embodiments, the signal quality measurement may be based on observed bit-error-rate (BER), estimated best attainable BER, or estimates of PMD affecting the received signal. Such measurements may be performed in the receiver by altering the threshold level and sampling timing of a bit detector in the receiver and monitoring the resulting changes in BER.
It should be noted that modem communication networks are characterized by high transmission rates, and the trend is to increase the transmission rates more and more. Due to that, any equipment in such networks (and the measurement equipment in particular) is quite expensive and will become even more expensive.
One attempt to provide a cheap data quality measuring device for high bit rate signals is provided in the U.S. Pat. No. 6,501,573. The patent concerns an apparatus and a method for monitoring qualities of an optical signal. The apparatus is arranged for extracting the clock component of data from the optical signal transmitted and measuring the magnitude and then detecting the presence or absence of the signal and the error rate upon receiving the deteriorated signal due to the chromatic dispersion. The simple measuring apparatus has an optical coupler, a signal conversion means, a narrow optical detector, a super high frequency rectifier and a voltage detector. Still, the measuring circuit is a bit rate dependent circuit and is integrated in the data transmitter & receiver.
Similarly to the latter patent, U.S. Pat. No. 6,606,354 provides measurements of quality of a signal having a high bit rate, which is quite complex and expensive.
Yet another kind of methods provides monitoring and analyzing a signal composed from a number of communication channels, in order to estimate the number of channels presently active in the composite signal. Exact solutions of this task usually comprise expensive equipment such as a spectral analyzer or a plurality of precise filters.
U.S. Pat. No. 6,072,601 describes an optical fiber amplifier capable of determining a number of optical channels actually inputted to it, thereby allowing, through self-control, both optimum operation conditions in accordance with the number of transmission channels, and the maintenance of the transmission performance. The optical fiber amplifier comprises an optical signal amplifier and a control circuit. The control circuit section comprises a branch circuit that branches off and extracts a portion of the transmitted optical signal power, and a channel counter that inputs a portion of the branched optical signal power and based on that counts the number of channels of the transmitted optical signals.
However, channel counters, especially for high frequency/high bit rate systems, are quite expensive. This applies not only to optical, but to any wired or wireless communication systems.